Field labels should be sentence case

it looks like we infer field labels in a way that breaks our UI text style guidelines.

The following discussion from !59518 (merged) should be addressed:

• @msedlakjakubowski started a discussion: (+1 comment)

  

  To the backend reviewer: I see that the event descriptions sit in https://gitlab.com/gitlab-org/gitlab/blob/master/app/helpers/services_helper.rb but I don't know where their names are defined.

  To follow Pajamas, the names should be in sentence case, so for example: Merge request, Confidential issue, etc.